Henderson Hasselbalch Calculator

It links a buffer's pH to the acid's pKa and the balance of its two forms: pH = pKa + log10([A-] / [HA]), where [A-] is the conjugate base and [HA] is the weak acid. Rearranged, it also gives the ratio needed for a target pH as [A-]/[HA] = 10^(pH - pKa). It is the working formula for designing and predicting buffers.

When the conjugate base and weak acid concentrations are equal, their ratio is 1 and log10(1) is 0, so the pH lands exactly on the pKa. This is also the point of maximum buffering capacity, where the buffer resists added acid or base most strongly.

Switch to the target pH mode and enter a pKa and the pH you want. The tool computes the ratio as 10 raised to the power pH minus pKa. A pH one unit above the pKa needs a 10 to 1 base to acid ratio, and one unit below needs 1 to 10.

Molarity, mol/L, is standard, but because the equation depends only on the ratio of the two concentrations, any consistent unit cancels and gives the same pH. You can even use moles or millimoles as long as both boxes use the same unit and the volume is shared.

It is most reliable within about one pH unit of the pKa, roughly a 1 to 10 up to 10 to 1 ratio, where a buffer actually works. Far outside that window, or at very low concentrations, it drifts because it uses formal concentrations instead of true activities and ignores water's own contribution to pH.

The pKa is the anchor of the equation; without it there is no reference point to add the log term to. In Concentrations mode you also need both concentrations, and in target pH mode you need the pH, but the pKa is mandatory either way.

Each concentration must be a positive number. Zero or negative values are rejected, and you must supply both the base and the acid or neither, since a single concentration cannot define a ratio.

No. You supply the pKa, which you look up for your acid, such as 4.76 for acetic acid or about 6.1 for the carbonic acid system in blood. The tool then handles the buffer arithmetic around that value.

Yes. The logarithm and power calculations run entirely in your browser, so the pKa, pH and concentrations you type are never uploaded and the tool works offline once loaded.